1. Field of the Invention
The invention relates to information signal communicating apparatus, method, and system and, more particularly, to a modulating process of an information signal which is transmitted through a transmission path.
2. Related Background Art
Generally, in a spread spectrum communication by the direct spread system, a spread modulation is performed to an information signal (baseband signal) to be transmitted by using a spread code series such as a pseudo noise code (PN code) or the like, thereby forming a spread modulation signal of a band width that is extremely wider than that of the baseband signal. The spread modulation signal is further converted into a radio frequency (RF) signal by using a PSK (phase shift keying), FSK (frequency shift keying), or the like and is transmitted to the reception side.
On the reception side, the received RF signal is demodulated to the spread modulation signal before transmission. Subsequently, the reception side obtains a correlation with the spread modulation signal by using the spread code series used on the transmission side (namely, a reverse spread modulating process is performed) and converts the spread modulation signal to the baseband signal before transmission.
According to such a spread spectrum modulation system, a band width of the transmission signal is extremely wider than that of an information signal. Therefore, under a condition such that the band width of the transmission path is constant, an amount of information to be transmitted is fairly smaller than that of the conventional radio communication system and only a low transmission speed can be realized.
As a method of solving the above problem, there is a method called a code division multiplex. According to this method, the information signal to be transmitted is divided into a plurality of parallel data and each data is spread modulated by a plurality of different spread code series. Subsequently, those spread modulation data is multiplexed and is converted into the RF signal and is transmitted. By using this method, a high speed data transmission can be realized even under a condition such that the band width of the transmission path is constant.
However, the above code division multiplex has the following problems. For example, as the multiplex number increases in order to transmit a larger amount of information, a phenomenon called a multipath fading is likely to occur. According to such a phenomenon, when a direct wave and a reflection wave whose phase differs from that of the direct wave are interfered, an electric power level of the reception signal deteriorates. Due to an influence by such a phenomenon, when the multiplex number is increased, an amount of errors which are caused in the transmission signal increases, so that there is a problem such that a communication fault or a communication stop occurs.
Further, under an environment of the transmission path such that noises which are mixed into the transmission signal change with the elapse of time, it is necessary to set the multiplex number in accordance with the environment of the transmission path. However, since it is difficult to detect the noises which are mixed during the communication, it is very difficult to control the information amount in accordance with the environment of the transmission path.